Blackberry showing 4G and BB logo but constant Call Failed

Hello,

I have an unlocked blackberry 9900 from at&t and currently using it in Peru with a carrier that offers 4G network (I have the 4G logo) and I would like to take this opportunity to say that before the upgrade my blackberry sometimes displayed the 4G and blackberry logo but when placing calls a Call Failed message would appear and the call won't even connect, the same happened when trying to send bbm messages, a red cross appeared after a long while. Is my device broken or its and OS problem? (this reason made me look around for other OS versions) All this happened on official os 7.1.391 from rogers.
I have tried looking up all of this on the internet for about 2 months now and the problem appeared at any time with no reason. Another issue was that the bb logo disappeared leaving only the 4G but still cannot use bbm.
If some one has experienced this or just knows whats happening with my blackberry please help me!